THE EMBRYOLOGY OF THE OPOSSUM IS.'! 



The crawling foetuses of stages 34 and 35 are so ceaselessly 

 active that I have not so far devised a successful method of 

 testing them for reflexes to tactile stimulation. But the 

 crawling movements themselves are interesting. These begin 

 with the lateral flexion of the head and neck followed closely 

 by the forward movement of the ipsilateral foreleg. The 

 head then starts a flexion toward the opposite side while the 

 claws of the forefoot close in a clasping movement and the 

 foreleg is moved backward in an effective, propulsion stroke. 

 The caudal end of the body is then flexed as a whole toward 

 the side on which the effective stroke of the forelimb is being 

 executed. This constitutes a complete cycle. A similar cycle 

 is initiated on the contralateral side when the head moves in 

 that direction. The whole cycle seems to represent a wave 

 of contraction which starts at the anterior end of the body on 

 one side and travels posteriorly, the similar wave of the op- 

 posite side starting before the first wave has progressed be- 

 yond the anterior limb. This is a typical Coghill ( '29) 

 S-reaction except for the precocious individuation in the 

 forelimb. 



The attachment to the nipple. When the young enters the 

 pouch he finds himself in a maze of tangled, curly hairs among 

 which he must locate one of thirteen, tiny mammary nipples 

 about the size of the head of a common straight pin. The 

 sensory equipment which he has to assist him in this search, is 

 very scanty indeed, as will be described below. And perhaps 

 the most amazing thing that he does is to succeed in this 

 search ; but once in the pouch, he is almost certain to do so. 

 T have watched them do it many times, and I do not yet know 

 what clues they use certainly not sight, possibly smell, 

 possibly cutaneous sensations of warmth, moisture, and touch. 

 The head is moved in wide excursions to right and left, and 

 when the snout finally touches the soft, moist, and warm 

 nipple, the wandering stops and the young promptly sucks the 

 nipple into its mouth. 



The sealing of the lateral portions of the lips by means of 

 epitrichial cells in stage 34 leaves the mouth large inside, 



